A person in California is believed to be the first person to contract coronavirus despite not having travelled outside the United States or coming into contact with a known patient.
The chilling development, reported Centres for Disease Control and Prevention, is the first sign the disease may be spreading within a community in the US.
The Solano County patient had "no known exposure to the virus through travel or close contact with a known infected individual," the California Department of Public Health said.
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The case was picked up in the public health system by "astute clinicians," the CDC said.
Health officials are now urgently tracking the patient's prior movements.
"At this time, the patient's exposure is unknown," the CDC said. "It's possible this could be an instance of community spread of Covid-19, which would be the first time this has happened in the United States."
The CDC told KCRA that "unprecedented, aggressive efforts" had been undertaken to contain the spread and mitigate the impact of the virus in the US.
President Donald Trump earlier gave a press conference in which he downplayed the threat of the virus to the US.
He also appointed Vice President Mike Pence to lead the country's response to the virus.
GROWING GLOBAL PROBLEM
South Korea has confirmed almost 300 new coronavirus infections as the country struggles to deal with the growing crisis.
The South Korean military announced additional infections among its troops, with 20 cases on its bases and some 9570 people in isolation.
The US military, which has 28,500 troops in South Korea, confirmed the first infection of an American soldier, a 23-year-old man based at Camp Carroll near Daegu.
With Brazil confirming the arrival of Latin America's first case, the virus has a toehold on every continent but Antarctica.
The latest countries to confirm infections include North Macedonia, Pakistan, Finland, Kuwait, Lebanon, Sweden and Georgia.
The disease has now spread to at least 45 countries, with many implementing emergency plans and travel restrictions on the worst-affected places.
Australia activated its emergency response plan yesterday amid rising concerns of a potential coronavirus pandemic.
Stock markets around the world have been savaged, with more than $US1.7 trillion wiped off US exchanges in two days alone.
Meanwhile. the World Health Organisation (WHO) reported that the number of new cases reported outside China exceeded the number of new cases in China for the first time.
On Tuesday, 411 new cases of the Covid-19 disease were reported in China, while 427 were reported outside the country, WHO said.
"The sudden increases of cases in Italy, the Islamic Republic of Iran and the Republic of Korea are deeply concerning," WHO director-general Tedros Adhanom Ghebreyesus said on Wednesday.
Countries across the world are now scrambling to prevent the spread of the virus, which was first detected at a seafood market in Wuhan, China in December.
More than 81,000 people have been infected while at least 2770 have died.
South Korea announced more than 1000 people had been infected – by far the largest outside China – and 12 deaths.
Italy, the hardest hit country in Europe, reported 400 people have now been infected, a rise of 80.
Iran officially announced a total of 19 deaths and 139 infections, including the country's deputy health minister. The actual number is thought to be higher.
The CDC warned the American public to prepare for an outbreak of the disease.
But before he flew home from India on Tuesday, Trump said the coronavirus situation was "very well under control in our country".
The US administration has asked Congress for an additional $2.5 billion to speed development of a vaccine, support preparedness and response activities, and to gather needed equipment and supplies.
IS IT A PANDEMIC YET?
Around the world cleaning crews scrubbed down everything from money to buses and military bases and hotels were on high alert for the disease.
Worries about the economic impact are growing, with factories idle and tourism crippled.
The dramatic surge of infections in Italy, Iran and South Korea has prompted calls for the virus to be declared a pandemic.
WHO defines a pandemic as a situation where "the whole world's population" is likely to be exposed to an infection and potentially "fall sick".
The declaration would prompt countries to step up their preparedness plans, but the WHO said it could also cause unnecessary panic.
"We should not be too eager to declare a pandemic," Tedros said on Wednesday, stressing that such a declaration could "signal that we can no longer contain the virus, which is not true".
"We are in a fight that can be won if we do the right things."
But Tedros insisted WHO would not hesitate to declare a pandemic if it was "an accurate description of the situation".
"I am not downplaying the seriousness of the situation, or the potential for this to become a pandemic, because it has that potential," he said.
"All countries, whether they have cases or not, must prepare for a potential."
VIRUS COULD STICK AROUND FOR MONTHS
The coronavirus epidemic is believed to have peaked in China between January 23 and February 2.
But Bruce Aylward, the leader of a joint WHO-China mission of experts, said it could be a threat "for months".
He warned countries they needed to do more to get ready.
"Think the virus is going to show up tomorrow. If you don't think that way, you're not going to be ready," he said.
"This a rapidly escalating epidemic in different places that we have got to tackle super fast to prevent a pandemic."
In the US, the CDC warned the American public to prepare for an outbreak.
"It's not so much a question of if this will happen anymore, but rather more a question of exactly when this will happen, and how many people in this country will have severe illness," senior official Nancy Messonnier said.
She said schools could help contain the spread by closing down or using "internet-based teleschooling" while businesses could "replace in-person meetings with video and telephone conferences".
On a larger scale, cities may need to cancel mass gatherings, she said.